Unless you have chicks that have been bred for one of the sexing tricks (wings or color), you just have to wait for them to either show cockerel traits or not. Maybe you can find vent sexer training but it's not easy to come by.
You mean when they get their second set of neck feathers, right? Baby feathers and rooster plumage are very different things. The long, skinny, shiny neck and saddle feathers exclusive to males start to come in around three months of age.
There are many different tricks of the trade. Usually at about 6 weeks the roosters will have red comb & wattles & the girls won't. If you have one that is iffy you can always post it on here and get people's opinions. Once you have had a batch or 3 you will start to be able to tell the differences easier and at least have a pretty good educated guess.
OK I have some pictures of my Buffs. I have read all kinds of posts on BYC and I am still confused. I bought some chicks in January that were born in November, and are just about 6 months old now. They were all supposed to be hens. Yesterday morning at one started to crow . There are 18 babies and several have the large comb and wattles.
Here is one I suspect is Roo: